Recently, miniaturized chemical instruments have been fabricated using micromachining techniques. These "microchip" devices have been used to perform liquid phase separations such as electrochromatography and electrophoresis and mixing of reagents in integrated microreactors for chemical reactions.
Among their many advantages, microchips allow increased speed of analysis and reduced reagent and sample consumption over conventional bench-scale instruments. In addition, integrated devices provide significant automation advantages as fluidic manipulations are computer controlled. These integrated devices are now being referred to as "lab on a chip" technologies, as the operations of a complete wet chemical laboratory could potentially be integrated.
Many chemical analysis tools that are used in modern laboratories are not presently miniaturized and many are not thought to be amenable to miniaturization in the immediate future. Although the microchip technology is quite powerful, there are situations where after processing on a microchip, further chemical interrogation is desired off of the microchip. A convenient way to transport fluids from microchips to other devices is to generate droplets that can be directed to specific locations for collection and/or analysis. Ink jet technologies are in wide spread use in printer products and are currently used to distribute liquid borne chemicals with spatial selectivity. Early ink jet formation methods included continuous stream ink jets, impulse ink jets, electrostatically generated ink jets. See, for example, R. D. Carnahan, and S. L. Hou, IEEE Trans. Ind. Appl., IA-13, 95 (1977). The first two methods use acoustic energy to form droplets and the latter uses electrostatic forces. Most modern ink jet printers utilize thermal energy to form droplets where bubble formation in the ink essentially provides the acoustic driving force to launch a droplet. See, for example, R. A. Askeland, W. D. Childers, and W. R. Sperry, Hewlett Packard Journal, August, pg. 28 (1988). Charged droplets and even molecular ions can be generated from liquids using the technique called "electrospray."
Electrospray is often used as a method for generating gas phase ions from solution for subsequent mass spectral analysis. Electrospray ionization is a soft ionization technique whereby species that are ionic in solution are transferred to the gas phase. The sample solution is dispersed as an electrically-charged aerosol and following solvent evaporation and disintegration of the droplets into smaller droplets, gas-phase ions are eventually produced. In the past, electrospray ion sources have employed needles or capillary tubes for spraying ion sources.
Essentially no fragmentation accompanies the ionization process and multiply-charged ions are typically produced from high mass polymers such as peptides, proteins, DNA and various synthetic polymers. Thus, electrospray ionization mass spectrometry is an effective means to provide primary and secondary structural analysis of polymeric materials.